Cooling system with integral thermal energy storage

ABSTRACT

A cooling system is provided which includes a primary heat exchanger including heat exchange pipes filled with a phase change material comprising water. The heat exchange pipes are in heat transfer relation with a coolant fluid, which, upon cooling, is transferred to a separate liquid to air heat exchanger for conversion into cool air, for example, for use in air conditioning or refrigeration.

BACKGROUND OF THE INVENTION

The present invention relates to a cooling system, and moreparticularly, to a cooling system which utilizes phase change materialsfor the storage and release of thermal energy.

It is known that a large amount of electric power is consumed by thecooling of residual and commercial buildings, especially during daylighthours. Overall, a large imbalance in electric power usage exists duringdaylight hours, due primarily to the amounts of power consumed byindustry, businesses, and public transportation. To compensate for theextensive day time use of electric power, utility companies providegenerating capacity sufficient to supply day time usage, leaving unusedcapacity available for the night hours.

A need has arisen in the art for a cooling system which can provide moreefficient cooling and which can make effective use of energy duringoff-peak hours. Cooling units are known which utilize phase changematerials to provide more effective cooling. A cooling unit utilizing aphase change material is described in my U.S. Pat. No. 5,765,389. Thecooling unit includes heat exchange conduits including a coolant fluidtherein which are positioned in heat transfer relationship with a phasechange material such as a melt mix polymer or linear crystalline alkylhydrocarbons.

However, there is a need for an improved cooling system which can reducecommercial and residential energy costs through the use of moreeffective phase change materials.

SUMMARY OF THE INVENTION

Embodiments of the invention meet that need by providing a residentialand commercial cooling system such as an air conditioning unit whichprovides cooling by using a phase change material contained in aplurality of heat exchange pipes which are in heat transfer relationwith a coolant fluid.

According to one aspect, a residential and commercial cooling system isprovided which comprises a primary heat exchanger comprising an outershell, and a plurality of heat exchange pipes positioned in the shell,where the pipes contain a phase change material therein. The systemfurther includes at least one cooling element in the shell. Preferably,the cooling system includes first and second cooling elements positionedat the top and bottom of the primary heat exchanger.

The cooling system further includes a coolant fluid in contact with theheat exchange pipes, and an inlet and an outlet for the transfer of thecoolant fluid to and from the heat exchanger.

The system further includes a separate liquid to air heat exchangerincluding an inlet and outlet for the transfer of coolant fluid to andfrom the liquid to air heat exchanger; and an inlet and outlet for thetransfer of air to and from the liquid to air heat exchanger.

In one embodiment, the phase change material comprises water/ice. Thephase change material may further comprise from about 1 to about 5% byweight polyvinyl alcohol.

In an alternative embodiment, the phase change material comprises awater/urea phase change material comprising from about 60 to 80% byweight water and from about 20 to 40% by weight urea.

The coolant fluid for use in the system is selected from the groupconsisting of ethylene glycol, propylene glycol, and glycerine. Thecoolant fluid is contained in the cavity of the shell such that it comesinto contact with the heat exchange pipes filled with phase changematerial.

The heat exchange pipes comprise a metal selected from copper, stainlesssteel, and glass-coated steel. The heat exchange pipes have an outerdiameter of from about 0.5 to about 2.5 inches, and include a closuresuch as a cap at each end such that the phase change material is sealedtherein.

The cooling system preferably has a three-dimensional rectangular orcubic configuration with generally flat sides. A layer of insulation maybe included on the exterior surface of the shell. The insulation ispreferably vacuum panel insulation having an R value of about 50 to 60per inch of thickness.

The coolant fluid is cooled by the cooling element(s) which may befilled with a refrigerant such as freon. The cooling system ispreferably operated so that the coolant fluid (and the phase changematerial) is cooled from the bottom up. As the coolant is cooled,cooling of the phase change material is accomplished via direct contactof the coolant fluid with the metal heat exchange pipes containing thephase change material contained therein. When the cooling system is notin use, energy stored in the phase change material is transferred backto the coolant fluid such that the temperature of the fluid lowers andmay be transferred out from the heat exchanger to the separate liquid toair heat exchanger positioned externally from the primary heatexchanger, where the coolant fluid will be used to cool air, forexample, for air conditioning. Alternatively, the cooled coolant fluidcould be used in a refrigeration unit.

In one embodiment, the cooling system may include a solenoid valve inconjunction with the inlet or outlet to provide pulsatile flow of thecoolant fluid and improve cold transfer.

Accordingly, it is a feature of the invention to provide a coolingsystem which employs a primary heat exchanger comprising a plurality ofheat exchange pipes including a phase change material therein. These,and other features and advantages of the invention, will become apparentfrom the following drawings, detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial cross-sectional view of one embodiment of thecooling unit;

FIG. 1B is a top view of embodiment of the cooling unit shown in FIG.1A; and

FIG. 2 is a perspective view of a single heat exchange pipe including aphase change material therein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the cooling system use water as a phase change materialwhere the water in liquid form provides sensible heat and the water/icecombination provides a latent heat of about 80 calories per gram. Waterprovides advantages over the use of other conventional phase changematerials such as linear crystalline alkyl hydrocarbons as it has ahigher latent heat and higher sensible heat, is much lower in cost, andis readily available.

Referring now to FIGS. 1A and 1B, one embodiment of the cooling system10 is shown. As shown in FIG. 1A, the cooling system 10 comprises ashell 12 including a primary heat exchanger 14 therein which comprises aplurality of heat exchange pipes 16 including a phase change material 18therein. The heat exchanger 14 is used in conjunction with a separateliquid to air heat exchanger 32 which is capable of supplying thecooling requirements of a residential home.

The shell 12 is preferably rectangular or cubic in configuration andshould be of a sufficient height to be able to accommodate up to about75 gallons of coolant fluid. The shell 12 may be comprised of copper,stainless steel, or glass-coated steel. It should be appreciated thatthe shell and the heat exchange pipes should comprise the same material.For example, if the heat exchange pipes comprise stainless steel, theshell should also be comprised of stainless steel to provide optimumheat transfer and to avoid creating a battery effect between the twometals in solution.

As shown, the shell 12 has a flat exterior surface 20 which issurrounded by an insulation material 22. The insulation material 22preferably covers substantially the entire exposed exterior surface 20of shell 12. Preferably, the insulation material 22 has an “R” value ofat least about 50 to 60 per inch. Vacuum panel insulation suitable foruse includes vacuum panel insulation available from AcuTemp under thedesignation ThermoCor®. The coolant fluid is supplied to the shellduring manufacture of the heat exchanger and is filled close to the topof the shell as indicated by fluid level 48 as shown in FIG. 1A. As theshell is a closed unit, there is no need to add additional coolantfluid.

Suitable coolant fluids for use in the cooling unit include ethyleneglycol, propylene glycol, and glycerine, which are preferably mixed withwater. A preferred coolant fluid is a mixture of ethylene glycol andwater in approximately equal amounts.

An outlet line 26 allows cooled fluid to flow from the heat exchanger 14to a separate heat exchanger 32 for conversion to cooled air as will beexplained in further detail below. If desired, the outlet line 26 mayinclude a programmable solenoid valve 50 as shown. The solenoid valvemay be partially closed at regular intervals to provide pulsatile flowof coolant fluid to improve cold transfer. The solenoid valve can beprogrammed to vary both the amplitude and frequency in which the valveis partially closed to provide the desired pressure drop. Variations offrequency from 5 to 60 cycles per minute, and more preferably, fromabout 15 to 30 cycles per minute are desirable. The valve closure ispreferably regulated so as to create a pressure drop of at least 5 psi,and preferably up to about 50% of the available fluid pressure.

The cooling unit 10 further includes cooling elements comprising coils28 and 30 positioned at the top and bottom portions of the shell. Thecooling coils 28, 30 preferably contain a refrigerant therein such asfreon or fluorocarbon gas. These materials are preferably selected tohave a boiling point which is desirable to initiate freezing of thephase change material. In operation, evaporation and cooling of therefrigerant material takes place in the cooling coils similar to a freontype refrigeration or air conditioning system. The cooling unit furtherincludes a compressor 34 positioned externally from the heat exchanger14 which includes a condenser for returning the freon to liquid form forreuse as in a conventional refrigeration unit. The freon or otherrefrigerant is pumped through the cooling coils. As the refrigerantevaporates, it cools the coil, which then cools the coolant fluid in theprimary heat exchanger. The evaporated coolant gas is then returned tothe compressor, where the gas is converted to a liquid which is directedto the coils in the primary heat exchanger 14.

The cooling unit 10 may also include a timer (not shown) connected to apower supply (not shown) to control the power usage of the heatexchanger during designated time periods, e.g. turning off the powersupply during peak power usage hours, thus reducing energy costs.Suitable power sources include photovoltaic or wind chargers.

The heat exchanger 14 further includes a plurality of heat exchangepipes 16 including the phase change material 18 therein. As shown, theheat exchange pipes 16 are positioned vertically in the heat exchanger.The heat exchange pipes are preferably configured in the shell as shownin the top view of the water heater depicted in FIG. 1B and arepreferably held in position by a perforated metal screen (not shown)with holes therein which fit around the heat exchange pipes to hold themtogether as a unit.

Prior to being filled with the phase change material, the pipes arehollow and are comprised of a heat conducting material. The heatexchange pipes include a cap at each end for sealing the phase changematerial, which will be described in more detail below. While the pipesand phase change material are shown in cylindrical form, it should beappreciated that both the pipes and phase change material may also varyin shape. For example, the pipes and corresponding phase change materialmay be square or rectangular in shape.

A preferred phase change material for use in embodiments of theinvention is water, which has a melting/freezing temperature of 0° C.and a latent heat capacity of 80 calories/gram. Preferred for use in theinvention is pure water obtained by distillation or reverse osmosis.

Another suitable phase change material is a water/urea mixture having amelting/freezing temperature of about −12° C. and a latent heat capacityof about 70 calories/gram. Such a water/urea mixture preferablycomprises about 70% water and 30% by weight urea. In this embodiment, alower temperature refrigerant would be required for use in the coolantcoils to freeze the phase change material. Suitable lower temperaturerefrigerant materials include Freon materials having varied fluorinecontents and molecular weights, such as, for example, Freon® 22commercially available from Dupont.

Upon melting and freezing, the phase change material absorbs andreleases a large quantity of energy in the vicinity of itsmelting/freezing point. The phase change materials may be repeatedlyconverted between solid and liquid phases to utilize their latent heatsof fusion to absorb, store, and release heat during the phaseconversions at about 0° C. for pure water or about −12° C. forwater/urea mixtures.

The phase change material may further include from about 1 to 5% byweight of a polymeric thickening agent such as polyvinyl alcohol whichraises the viscosity of the water phase change material, reducing thelikelihood of leakage of water during the multiple cycles of melting andfreezing that take place during operation of the cooling unit.

The phase change material may further include nucleating agents such assilicon dioxide dry powders or silver iodide. Such agents may beincluded in amounts of about 1 to 10% by weight silicon dioxide or about0.05 to 0.5% by weight silver iodide to prevent super cooling in thesystem.

Referring now to FIG. 2, a single heat exchange pipe 16 for containmentof the phase change material 18 is shown which is initially hollow inform and may be formed from metals including, but not limited to,stainless steel, copper, and glass-coated steel. The outer diameter ofthe pipes may range from about 0.5 to 2.5 inches, more preferably, about1 to 2 inches, and most preferably about 1.5 inches. The wall thicknessof the pipe should be sufficient to withstand normal pressures duringoperation and is preferably from about 0.030 to 0.125 inches, and morepreferably, about 0.060 inches. The pipes 16 are preferably provided inlengths of about 27 inches, while the phase change material 18, onceplaced into the pipes, is about 24 inches in height, which allows atleast about 3 to 4 inches of empty space in the pipe for expansion ofthe phase change material when it freezes.

Prior to providing the phase change material 18 in the pipe, an end cap40 is applied to one end of the pipe and adhered thereto by a hightemperature thermosetting adhesive, by providing mating threads on thepipe and cap, or by soldering (where copper pipes are used).

The open end of the empty pipe 16 is then filled with a source of inertgas such as nitrogen or argon such that most of the oxygen in the pipeis purged. The phase change material is then poured into the pipe sealedat the bottom with cap 40 such that the water level is about 4 inchesbelow the top of the pipe. A second end cap 42 is then placed over theopen end of the pipe 16 and secured thereto in a conventional manner asdescribed above. The second end cap 42 includes a hole 44 which has beendrilled in the center of the cap which allows residual gas inside theheat exchange pipe to be vented as the phase change material freezes andexpands for the first time. This avoids the buildup of high pressurefrom the expansion of the phase change material which could potentiallycause swelling and/or rupture of the pipes.

The initial freezing and expansion of the phase change material shouldtake place prior to final assembly of the cooling system, i.e., prior toplacing the heat exchanger inside the shell. After the venting process,the cap 42 is then permanently sealed. The cap may be sealed with athreaded metal screw comprised of the same metal as the pipe, by the useof a high temperature thermosetting adhesive or by soldering (wherecopper pipes are used).

The filled heat exchange pipes are then assembled in a compactconfiguration consisting of approximately 24 rows with about 24 heatexchange pipes in each row (arranged in a rectangular or squareconfiguration). A perforated metal screen 36 having openings toaccommodate the pipes may be used at the top and bottom of the pipes tomaintain them in proper position. A thin metal strip may also beattached to the bundle of pipes to keep the pipes in place. The bundleof pipes including the phase change material therein is then insertedinto the shell of the cooling unit.

The coolant fluid contained in the cavity of the heat exchanger providesa fluid transfer medium which comes into contact with the water-filledheat exchange pipes for cooling. The water phase change material is indirect heat transfer contact with the inner surfaces of heat exchangepipes 16 so that, during operation, as the coolant fluid surrounds theheat exchange pipes, heat can be transferred from the phase changematerial 18 to the coolant fluid and vice versa.

The thermal energy supplied from the cooling system is delivered on aplateau of nearly constant temperature until the latent heat capacity isexhausted and electric power is supplied. If desired, lower costoff-peak electricity or a green source of energy such as solarphotovoltaic or wind driven devices can be used to supply the energyrequired to “charge” the phase change material, resulting in significantcost savings for consumers.

Referring again to FIG. 1A, the cooling system 10 operates in thefollowing manner. The cooling coils 28 and 30 positioned at the top andbottom portions of the shell include a refrigerant material therein suchas freon which is supplied from compressor 34. The temperature of therefrigerant material is controlled by a thermostatic valve (not shown)which controls the rate of coolant flowing through the top and bottomcoils such that a desired temperature differential is maintained betweenthe two coils and to allow freezing of the phase change material fromthe bottom up. By freezing the phase change material from the bottom up,the phase change material can expand into the empty space at the top ofthe heat exchange pipe without a build-up of pressure.

As the coolant fluid in the cooling system 10 is cooled by the coolingcoils (via evaporation of refrigerants in the coils), energy from thecoolant fluid is transferred from the metal heat exchange pipes 16 tothe water phase change material 18 contained therein, which begins tofreeze. Once the water reaches the freezing point (0° C.), the icecontinues to cool the system until heat is added to the system at whichpoint the coolant fluid becomes warmer than the (ice) phase changematerial and a melt cycle begins.

Thus, when the cooling unit 10 is not in operation, e.g., during peaktimes of power usage, the phase change material in the heat exchanger 14cools the coolant fluid, i.e., heat energy from the coolant fluid istransferred to the phase change material which lowers the temperature ofthe coolant fluid.

The cooled coolant fluid is then transferred from the primary heatexchanger 14 to the liquid to air heat exchanger 32 via outlet line 26.The heat exchanger 32 is a conventional liquid to air heat exchanger andincludes a return line 66 for the return of coolant fluid to the heatexchanger. The liquid to air heat exchanger 32 further includes a warmair inlet 68 and cold air outlet 70. As cooled coolant fluid enters theliquid to air heat exchanger 32, air entering the heat exchanger 32 iscooled by transport over the cooled pipes of the heat exchanger 32 suchthat the temperature of the air is reduced. The rate of heat transferdepends on the rate of air flow, the surface area of the pipe, andtemperature differentials. The cool air then exits through air outlet70.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the compositions andapparatus disclosed herein may be made without departing from the scopeof the invention, which is defined in the appended claims.

1. A residential and commercial cooling system comprising: 1) a primaryheat exchanger comprising: an outer shell; a plurality of heat exchangepipes positioned in said shell, said pipes containing a phase changematerial therein; at least one cooling element in said shell; a coolantfluid in contact with said heat exchange pipes; an inlet and an outletfor the transfer of said coolant fluid to and from said primary heatexchanger; 2) a liquid to air heat exchanger comprising: an inlet andoutlet for the transfer of said coolant fluid; and an inlet and outletfor the transfer of air.
 2. The cooling system of claim 1 includingfirst and second cooling elements positioned at the top and bottom ofsaid primary heat exchanger.
 3. The cooling system of claim 1 whereinsaid phase change material comprises water.
 4. The cooling system ofclaim 3 wherein said phase change material further comprises from about1 to about 5% by weight polyvinyl alcohol.
 5. The cooling system ofclaim 1 wherein said phase change material further comprises awater/urea phase change material comprising from about 60 to 80% waterand from about 20 to 40% by weight urea.
 6. The cooling system of claim1 wherein said coolant fluid is selected from the group consisting ofethylene glycol, propylene glycol, and glycerine.
 7. The cooling systemof claim 1 further including insulation on the exterior surface of saidouter shell.
 8. The cooling system of claim 7 wherein said insulation isvacuum panel insulation having an R value of about 50 to 60 per inch ofthickness.
 9. The cooling system of claim 1 wherein said heat exchangepipes comprise a metal selected from copper, stainless steel, andglass-coated steel.
 10. The cooling system of claim 1 wherein said heatexchange pipes have an outer diameter of from about 0.5 to about 2.5inches.
 11. The cooling system of claim 1 wherein each of said heatexchange pipes include a cap at each end such that said phase changematerial is sealed therein.
 12. The cooling system of claim 1 having arectangular configuration.
 13. The cooling system of claim 1 having acubic configuration.